feat(m5): notifications & activity log foundation

- Expand NotificationRepository (unread list paginated, count, markRead
  ownership-scoped); new ActivityLogRepository (create, findByProject
  paginated) with deterministic created_at+id ordering
- NotificationService (notifyOnEvent, resolveTargets for 8 event types,
  injectable SseBroadcaster for M8) + ActivityLogService (logActivity,
  listByProject)
- NotificationEvent/SseEvent/SseBroadcaster types in lib/types
- APIs: GET /api/notifications, POST /api/notifications/:id/read,
  GET /api/projects/:projectId/activity
- Screens: notifications list + MarkReadButton, NotificationBadge in
  Header, project activity page + ProjectNav link
- Unit tests for both repositories and services (28 new)

repositories/NotificationRepository.ts

@@ -9,10 +9,39 @@ export interface CreateNotificationInput {
   body: string | null;
 }
 
+interface NotificationRow {
+  id: number;
+  user_id: number;
+  project_id: number | null;
+  type: string;
+  title: string;
+  body: string | null;
+  read_at: string | null;
+  created_at: string;
+}
+
+function mapNotification(row: NotificationRow): Notification {
+  return {
+    id: row.id,
+    userId: row.user_id,
+    projectId: row.project_id,
+    type: row.type as NotificationType,
+    title: row.title,
+    body: row.body,
+    readAt: row.read_at,
+    createdAt: row.created_at,
+  };
+}
+
+export interface Paginated<T> {
+  items: T[];
+  total: number;
+}
+
+export const DEFAULT_PAGE_SIZE = 20;
+
 /**
  * notificationsテーブルへのデータアクセスを担うRepository。
- * M4ではメンバー追加通知のために create のみ実装する。
- * 一覧取得・既読化は M5 で拡張する。
  */
 export class NotificationRepository {
   constructor(private readonly db: SqliteDatabase) {}
@@ -42,4 +71,41 @@ export class NotificationRepository {
       createdAt: now,
     };
   }
+
+  /** ユーザーの未読通知一覧（作成日時降順・ページネーション） */
+  findUnreadByUser(
+    userId: number,
+    page: number = 1,
+    pageSize: number = DEFAULT_PAGE_SIZE
+  ): Paginated<Notification> {
+    const offset = (page - 1) * pageSize;
+    const items = this.db.query<NotificationRow>(
+      `SELECT * FROM notifications
+       WHERE user_id = @userId AND read_at IS NULL
+       ORDER BY created_at DESC, id DESC
+       LIMIT @pageSize OFFSET @offset`,
+      { userId, pageSize, offset }
+    );
+    const total = this.countUnreadByUser(userId);
+    return { items: items.map(mapNotification), total };
+  }
+
+  countUnreadByUser(userId: number): number {
+    const row = this.db.get<{ count: number }>(
+      'SELECT COUNT(*) AS count FROM notifications WHERE user_id = @userId AND read_at IS NULL',
+      { userId }
+    );
+    return row?.count ?? 0;
+  }
+
+  /** 通知を既読にする（本人所有の未読通知のみ） */
+  markRead(id: number, userId: number): boolean {
+    const now = new Date().toISOString();
+    const result = this.db.execute(
+      `UPDATE notifications SET read_at = @now
+       WHERE id = @id AND user_id = @userId AND read_at IS NULL`,
+      { now, id, userId }
+    );
+    return result.changes > 0;
+  }
 }